Light-emitting diode (LED) luminaires are often subject to high surge voltages having different origins. These surge voltages can be generated by lightning strikes, by transients during the switching on/off of inductive loads connected on the same network as LED luminaire, or by heavy industrial machines.
Additionally, in some situations, surge voltages can be generated by electrostatic charges which have accumulated on the luminaire body, and which are able to find a way to discharge through the LEDs and the LED driver within the luminaire housing to the mains connection lines. LEDs are particularly sensitive to high voltage strikes and could be damaged even without apparent damage to the LED driver. The consequence can be, some if not all, LEDs within a group of LEDs short circuiting as their dies have been destroyed by high voltages.
Such high surge voltages can be either in differential mode, where the high voltage appears across the mains supply, that is, between the Line and Neutral wires of the mains supply, or in common mode, where both the Line and Neutral wires of the main supply experience a high voltage transient relative to Ground. A combination of differential mode and common mode surges is also possible.
In order to protect a luminaire against high surge voltages, protection components are often provided in the input path of the luminaire, that is, in between the mains supply and a driver for an associated light-emitting diode module, and, often comprise metal oxide varistors (MOV) or gas discharge tubes (GDT) or a combination of both. These protection components are relatively efficient against differential mode surges as they are connected between Line and Neutral wires and absorb the energy of the surge having a current as high as several kA. Such protection components tend to provide good protection for the driver as it is the most susceptible to differential mode surges.
However, the problem is different for common mode surges. There are two cases situations depending on the IEC protective Class to which the luminaire is designed. In the case of a Class I luminaire, the presence of a protective earth (PE) connection allows protection components to be connected between the Line and PE or between Neutral and PE. These protection components tend to provide good protection against common mode surges. Moreover, series fuses inserted between the input and the protection component will shut off the circuit when the protection components are short circuited.
In the case of a Class II luminaire, there is no PE connection and no connections are allowed between the Line and Neutral wires of the mains supply and the luminaire housing for electrical safety reasons. This means that only differential protection components can be used for Class II luminaires. Nevertheless, this does not means that a surge cannot find a reliable return to Ground, as a return can inadvertently be provided through the metal frame or enclosure of the luminaire which is mounted on a metal pole which, in turn, is in direct contact with Ground.
In WO-A-2014/029772, an insulation member capable of sustaining the voltage levels of lightning strikes within the electrical path from a luminaire frame to Ground is provided for isolating the LED luminaire from Ground. Without return currents, there would be no damage to the luminaire. This is possible when the luminaire frame is mounted on a glass fibre or concrete pole, but is difficult to implement when the luminaire frame is mounted on a metal pole.
Additionally, it is also important to consider the potential danger for the LEDs due to high static voltages that can build up by electrisation of the luminaire frame due to the presence of heavily charged clouds during a lightning storm. In this case, it is also beneficial to avoid any high voltage differences between the LEDs and the luminaire frame.